This invention relates to cementitious compositions having good sag resistance and flow characteristics. The compositions of the present invention are especially suitable for use as dry-set mortars for the setting and grouting of tiles.
At present, ceramic tiles are set and grouted with dry-set mortars in most instances. A dry-set mortar is a cementitious composition which in the dry state is comprised of a blend of Portland cement, aggregate (e.g., sand, powdered limestone, and the like), and an organic thickening agent. Prior to use, the dry blend is mixed with water in sufficient amounts to provide a suitable viscosity for application.
Dry-set mortars such as the above which are typically used to set and grout tile in thin beds are disclosed in U.S. Pat. No. 2,934,932 and U.S. Pat. No. 3,243,307.
Rubber or polymer latex can also be added to dry blends such as the above to make a mortar. The resulting compositions are known as latex Portland cement mortars. The latex can contain additives in solution which would otherwise form part of the dry blend.
Dry-set mortars provide good workability, good resistance to staining, and sufficient bond strength. All of these properties are required in cementitious compositions which are employed for setting and grouting ceramic tile. In addition, such compositions must also have resistance to sag.
Sag resistance is a term used in the art and defined by a test method that is part of the American National Standard Specification for Dry-Set Portland Cement -- A 118.1. The term "sag resistance" is defined as a property or characteristic relating to the ability of a mortar to resist movement under load until a certain load level is reached. This property is very important in dry-set mortars and latex mortars because to be functional, a mortar on the one hand must be in viscous slurry or paste form, while on the other hand the mortar must also be capable of supporting the load imposed upon it by the tiles being set therewith. It is essential that the mortar provide support for the tiles without there being any appreciable sinking of the tiles into the mortar layer during that period in which the mortar undergoes hardening and sets. The property of sag resistance is especially important for wall applications because the mortar must hold the tile in relatively stationary vertical position while the mortar sets.
Most commonly, sag resistance has been provided in dry-set mortars by the use of asbestos, in the form of short fibers which are mixed with the mortars. Illustrative dry-set mortars having fibers to provide sag resistance are disclosed in U.S. Pat. No. 3,030,258 and the above-mentioned U.S. Pat. No. 3,243,307.
Although asbestos fibers and fibers in general have been found to be suitable for providing dry-set mortars with sag-resistant properties, these fibrous materials have drawn criticism for being harmful to human respiratory systems, and their use has recently been limited by national safety laws.
Surprisingly, it has now been discovered that if certain magnesium compounds are added in at least a certain amount to conventional hydraulic cementitious compositions, the resulting compositions possess good sag resistance which is at least comparable to that obtained with the use of asbestos fibers.